The invention is particularly applicable to cases where the enclosure is made of porcelain, e.g. vessels for circuit breakers. In enclosures of this type, there exist safety members that operate in the event of accidental excess pressure.
These members are either rated valves, or else membranes, capsules or plates which, when the pressure inside the enclosure reaches a given threshold, tear or break, thereby allowing the gas to expand to the outside and thus avoiding any danger of the enclosure being destroyed.
The increase in pressure inside the enclosure against which protection is required is due to an internal arc. Depending on the intensity of the arc, the time required for the pressure to reach the above-mentioned threshold value may vary from a few hundredths of a second for a high intensity arc to several seconds for a low intensity arc. When the time is relatively long, it is possible to avoid major damage to the active portions of the apparatus by using additional means to act quickly to tear the membrane. That is the object of the present invention.